Hitherto, considerable efforts have been exerted to heightening the sensitivities of silver halide photographic materials. The sensitivity of a silver halide photographic material depends on both the latent image forming efficiency, including the light absorption and spectral sensitization factors of silver halide grains, and the minimum latent image size.
With the intention of increasing the light absorption factor of silver halide grains, number of arts as described below have so far been proposed.
For instance, the art of tabular emulsion grains having a high aspect ratio as disclosed in U.S. Pat. No. 5,494,789 enables an increase in the quantity of dye adsorption per grain through an increase of grain surface area; as a result, an increase in the light absorption factor becomes possible. There is however a limit to the increase in grain surface area by the heightening of aspect ratio, and so the increase in light absorption factor per grain requires an increase of grain size also.
Other examples of a method for increasing the per grain surface area include the method of making holes in a certain part of each grain as described in JP-A-58-106532 and JP-A-60-221320 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and the method of forming ruffled grains as described in U.S. Pat. No. 4,643,966. However, the grains according to those methods have unstable forms, so that it is quite difficult to put them to practical use.
In addition, U.S. Pat. No. 5,302,499 discloses an increase in light absorption factor by the formation of a layer structure which can optimize the spectral sensitization characteristics and the grain thickness. However, the increase in light absorption factor by such a method is of the order of 10% at the greatest.
On the other hand, it is thought that the light absorption factor of sensitizing dyes are increased to promote the efficiency in transferring the energy of light to silver halide; as a result, the heightening of spectral sensitivities is achieved.
However, there is a limit to the quantity of sensitizing dyes adsorbed to the surfaces of silver halide grains, and it is difficult to make sensitizing dyes be adsorbed to the grain surfaces in a quantity greater than that required for saturating the grain surfaces with the sensitizing dyes adsorbed in a monolayer. Accordingly, the absorption factor of the photons incident upon each silver halide grain in the spectrally sensitized region is very low under the present conditions.
The methods proposed for solving the aforementioned problems are described below:
The adsorption of a cationic dye as the first layer and an anionic dye as the second layer through th e electrostatic force of attraction was proposed by P. B. Gilman, Jr. et al. in Photographic Science and Engineering, vol. 20, No. 3, p. 97 (1976).
The method proposed by G. B. Bird et al. in U.S. Pat. No. 3,622,316 consists in that the silver halide adsorbed by a plurality of dyes in a multilayer is sensitized through the contribution of the excitation energy transfer of Forster type.
The spectral sensitization through energy transfer from luminescent dyes was proposed by Sugimoto et al. in JP-A-63-138341 and JP-A-64-84244.
R. Steiger et al. tried the spectral sensitization through energy transfer from gelatin-substituted cyanine dyes in Photographic Science and Engineering, vol. 27, No.2, p. 59 (1983).
Ikegawa et al. carried out the spectral sensitization through energy transfer from cyclodextrin-substituted dyes in JP-A-61-251842.
The dyes in which two separate chromophores are coupled via covalent bonds without forming a conjugated system, or the so-called coupled dyes, are described, e.g., in U.S. Pat. Nos. 2,393,351, 2,425,772, 2,518,732, 2,521,944 and 2,592,196, and European Patent No. 565,083. Therein, however, those dyes did not make it their aim to heighten the light absorption factor. With respect to the dyes with a positive aim of increasing the light absorption factor, G. B. Bird, A. L. Borror, et al. disclose in U.S. Pat. Nos. 3,622,317 and 3,976,493 the sensitizing dyes of the coupled dye type which have a plurality of cyanine chromophores in a molecule, and therein the increase of the light absorption factor by adsorption of those dyes and the resultant sensitization through contribution of the energy transfer are proposed. Also, Ukai, Okazaki and Sugimoto proposed in JP-A-64-91134 the binding of at least one substantially non-adsorptive dye having at least two sulfo and/or carboxyl groups to a spectral sensitizing dye capable of being adsorbed to silver halide grains.
In addition, M. R. Roberts et al. proposed in U.S. Pat. No. 4,959,587 the spectral sensitization by the use of cyanine dye polymers.
As mentioned above, many proposals have been advanced on the method of increasing the light absorption factor, but all of them are insufficient in sensitivity heightening effect and attended by problems of causing an increase in intrinsic desensitization, retarding the development and so on.
Thus, novel spectral sensitization arts which enable the sensitivity to be heightened through an increase in the light absorption factor of silver halide photographic materials have been searched for.
The silver halide-utilized photographic materials have underwent a growing development in recent years; as a result, color images of high quality can be obtained simply and easily at present. Even now, the proposal of improved arts is continued in large number. In particular, various arts of using the so-called high silver chloride content grains having a high silver chloride content (more specifically, the term "high silver chloride content grains" as used hereinafter refers to the grains having a silver chloride content of no lower than 50%) have been proposed with the intention of simplifying and speeding up the development-processing. The use of high silver chloride content grains has advantages in that it can increase the developing speed, heighten the reusability of processing solutions, and so on. Therefore, the sensitive materials of the type which use high silver chloride content grains now came to prevail among the sensitive materials for printing use, including color photographic printing paper.
The high silver chloride content grains have a tendency to become grains having (100) faces as their outer surfaces (hereinafter referred to as "(100) grains") when they are formed under ordinary condition, and practically used grains also have a cubic crystal shape. In recent years, (100) grains having tabular crystal shapes have also been developed, and they have advantages, e.g., in that they can undergo effective spectral sensitization and provide a great covering power after development because of their great specific surface area (the surface area/volume ratio). Such tabular grains are disclosed, e.g., in U.S. Pat. Nos. 5,320,938, 5,264,337 and 5,292,632.
However, (100) high silver chloride content grains had a drawback of tending to be fogged, compared with silver bromide grains of common use. In order to remedy this drawback, the grains having a high silver chloride content and (111) faces as their outer surfaces (hereinafter referred to as "(111) high silver chloride content grains") were used. For instance, the use of such grains is disclosed in JP-A-6-138619.
The production of (111) high silver chloride content grains requires special measures. For instance, Wey discloses in U.S. Pat. No. 4,399,215 the method of producing tabular high silver chloride content grains in the presence of ammonia. According to this method, it is difficult to produce grains having sizes small enough to be used practically. This is because the use of ammonia compels silver chloride grains, which originally have high solubility, to be produced under a condition that their solubility is further heightened. In addition, the foregoing method has a disadvantage in that the grains are produced under a high pH condition (pH 8-10) to have a tendency to be fogged. Maskasky discloses in U.S. Pat. No. 5,061,617 the (111) high silver chloride content grains produced in the presence of thiocyanates. Similarly to ammonia, the thiocyanates increase the solubility of silver chloride. For the purpose of forming the high silver chloride content grains having (111) faces as outer surfaces without heightening the solubility, the methods of using additives (crystal habit control agent) during the grain formation are known. The following are examples of such a method:
______________________________________ Patent gazette Crystal habit control agent Inventor ______________________________________ U.S. Pat. No. 4,400,463 azaindenes Maskasky + thioether peptizer U.S. Pat. No. 4,783,398 2,4-dithiazolidinone Mifune et al. U.S. Pat. No. 4,713,323 aminopyrazolopyrimidine Maskasky U.S. Pat. No. 4,983,508 bispyridinium salt Ishiguro et al. U.S. Pat. No. 5,185,239 triaminopyridine Maskasky U.S. Pat. No. 5,178,997 7-azaindole compounds Maskasky U.S. Pat. No. 5,178,998 xanthin Maskasky JP-A-64-70741 dyes Nishikawa et al. JP-A-3-212639 aminothiol Ishiguro JP-A-4-283742 thiourea derivatives Ishiguro JP-A-4-335632 triazolium salts Ishiguro JP-A-8-227117 monopyridinium salts Ozeki et al. ______________________________________
Although the development of techniques has been progressed as mentioned above, the crystal habit control agent used during the grain formation is known to remain on the grain surface after the grain formation, thereby inhibiting the adsorption of spectral sensitizing dyes to the grain surface. Accordingly, spectral sensitizing dyes which are weak in adsorbing power cannot confer sufficient spectral sensitivities upon the grains.
As a means to make up for the spectral sensitivities, the method of using streptocyanine dyes has hitherto been known. These dyes can adsorb in a large quantity because of their small adsorption area per molecule; as a result, the light absorption factor per emulsion grain can be improved. For instance, such a method is disclosed in JP-A-9-127637. However, the dyes used therein cannot ensure a satisfactory rise in spectral sensitization efficiency although they can increase the quantity of absorbed light. Therefore, it has been desired to introduce improvements in the foregoing method in the case of silver chloride grains which are weak in dye adsorbing power, especially the grains formed using a crystal habit control agent.